Electrochromic Materials and Devices and Electrochromic Conducting Polymers
Electrochromic materials change color upon application of a voltage, generally a small (<5 V) DC voltage. The “color” change may be in the visible spectral region, but it may also be in the near infrared (NIR), infrared and microwave spectral region. Electrochromic devices may be transmissive-mode, in which light passes through the device and is modulated by the device, and reflective-mode, in which light is reflected off the device and also modulated by the device. Electrochromic devices may be used in windows, rear view automobile mirrors, flat panel displays, variable emittance materials for spacecraft application, and infrared camouflage.
The change in color of an electrochromic material is usually due to a reduction/oxidation (“redox”) process within the electrochromic material. Electrochromic materials active in the visible spectral region include metal oxides, such as tungsten, molybdenum, nickel and tantalum oxides, showing a transition from highly colored to near transparent depending on the potential (voltage) applied to them.
Another class of electrochromic materials are conducting polymers. Redox of a conducting polymer, which changes its color as well as conductivity, is usually accompanied by an inflow or outflow of counterions in the conducting polymer known as “dopants”. Common dopant counterions include ClO4− and BF4−. As examples, the conducting polymer poly(pyrrole) is dark blue and conductive in its oxidized (“doped” or “colored”) state and pale-green in its reduced (“de-doped” or “undoped”) state, and the conducting polymer poly(aniline) is nearly transparent in its reduced state, transitioning to green or dark green in its oxidized state. An electrochromic material is said to be “anodically coloring” if application of a positive voltage to it causes it to transition to a colored or dark state, and “cathodically coloring” if application of a negative voltage causes it to transition to a colored or dark state. Poly(pyrrole) and poly(aniline) are anodically coloring polymers.
The most convenient and common method of synthesis of conducting polymers for electrochromic uses is electro-polymerization from a solution of the monomer directly onto a conductive, transparent substrate, such as indium-tin-oxide (ITO) on glass, poly(ethylene terephthalate) (PET, “Mylar”) or other transparent plastic substrate. The electro-polymerization may be carried out using a constant applied potential (potentiostatic mode), a potential sweep (potential sweep mode) or other applied potential programs. Thus, e.g., poly(diphenyl amine) may be electrochemically deposited onto ITO/glass or ITO/PET from a 0.05 M solution of the monomer in acetonitrile at about +0.8 V (potentiostatic mode).
A common transmissive-mode electrochromic device is fabricated by depositing an electrochromic material on a conductive, transparent substrate, such as ITO/glass or ITO/PET, forming the active or working electrode. A similar substrate, ITO/glass, comprises the opposing or counter electrode. A liquid, solid or gel electrolyte is disposed as a layer between the two electrodes or incorporated into the polymers. The active electrochromic material on the working electrode may be switched to a dark “colored” or a less colored “bleached” state, depending on the voltage applied to it in this 2-electrode device, thus modulating the transmission through the device. A common reflective-mode electrochromic device may be fabricated in a similar fashion, with the difference that, in place of the transparent, conductive substrate, an opaque, conductive substrate, such as Au deposited on a microporous membrane, may be used. The counter electrode in such a device may be a similar conductive substrate disposed behind the working electrode. Such a reflective mode device is described in U.S. Pat. No. 5,995,273 (1999) and U.S. Pat. No. 6,033,592 (2000), issued to Chandrasekhar (collectively, the “Chandrasekhar IR patents”).
In the operation of such devices as described in the preceding paragraph, a voltage is applied to the working electrode. As an example, if the active electrochromic material thereon is anodically coloring, then a positive voltage will cause it to transition to a colored state. In the case of a conducting polymer, a corresponding inflow of counterions, in this case anions, will occur into the polymer.
In all 2-electrode electrochromic devices, at the same time that the working electrode experiences a (+) voltage, the counter electrode experiences the identical (−) voltage, and vice versa. An electrochemical reaction will then need to occur at the counter electrode to balance the charge transfer corresponding to the reaction occurring at the working electrode; the availability of a suitable counter electrode reaction is vital to the reversible functioning of the electrochromic device. In the case where the counter electrode substrate is bare or naked, i.e. it does not have an electrochemically active material such as an electrochromic material deposited on it, the likely electrochemical reaction that occurs is reduction of impurities present in the electrolyte, including, by way of example, dissolved gases (including oxygen); in the case of dissolved oxygen, species such as the superoxide ion or radical oxygen species may then be generated which have lifetimes as long as 20 seconds and which oxidatively or reductively degrade the active electrochromic material present on the other electrode (Menon et al., 1998) (Chandrasekhar IR patents, Chandrasekhar et al. 2002, Chandrasekhar et al. 1987). In such a circumstance, the overall electrochemical processes occurring within the electrochromic device are said to exhibit poor reversibility. This leads to a number of detrimental results, e.g. much more rapid degradation of the active electrochromic material and much slower electrochromic switching time.
Anodically-Coloring Conducting Polymers
Anodically-coloring conducting polymers described include poly(aniline), poly(pyrrole) as well as the structurally related series comprising poly(diphenyl amine), poly(4-amino-biphenyl) (Dao and coworkers (Guay et al., 1988, 1989, LeClerc et al., 1988, Nguyen et al., 1990)) and poly(N,N′-diphenyl benzidine) (Suzuki et al., U.S. Pat. No. 4,874,481 (1989)). These polymers show a color transition from nearly transparent in their reduced state to dark blue or blue-green in their oxidized state, with modest but consistent light/dark contrast, Delta %-Transmission between light/dark states at 575 nm being ca. 40%. Furthermore, the voltages required for their switching are relatively low, less than +1.5 V in many cases (in a 2-electrode-mode device with a bare ITO/substrate electrode serving as the counter electrode). An additional, key advantage of this series of poly(aromatic amine) polymers is that they are nearly transparent or, in some cases, completely transparent in their fully reduced state.
These polymers do however show a number of drawbacks, the most important of which is that, when incorporated into an electrochromic device without the presence of a suitable, complimentary counter electrode reaction, they display very slow light/dark switching times (up to 25 seconds) and modest contrast; they also then start to degrade after about 1000 cycles of light/dark switching. (Reasons for degradation include the lack of a counter-electrode reaction, resulting in impurities or water/oxygen in the electrolyte undergoing redox at the counter electrode; these may in turn generate harmful species, e.g. O2−, which further degrade the polymer). Nevertheless, these poly(aromatic amines) constitute an ideal set of anodically coloring electrochromic polymers, if they could be paired with a well-performing set of cathodically coloring electrochromic polymers in a single electrochromic device.
Cathodically Coloring Electrochromic Conducting Polymers and Structure-Performance Relationships Therein
In terms of cathodically coloring electrochromic conducting polymers, a number of these are described in the patent and journal literature. One of the first such polymers was poly(isothianaphthene) (first synthesized by Wudl and coworkers (Hotta et al., 1987, Patil et al., 1987) and with subsequent improvements in processing by Chandrasekhar et al., 1990), which transitions from a translucent blue-green in its oxidized state to a deep blue in its reduced state. Among its drawbacks was a relatively poor light/dark contrast (Delta % T typically 20% at wavelength of maximum absorption), asymmetric switching voltages (+1.3 V fully oxidized, −0.5 V fully reduced, all vs. Ag/AgCl), and rapid degradation (<200 cycles), i.e. poor “cyclability”.
A series of cathodically coloring polymers based on poly(3,4-ethylenedioxythiophene) (PEDOT) and on other polymers containing the thiophene moiety have been described by Groenendal et al., (2000), Sapp et al. (1998), Gazotti et al. (1998) and others. These yield a variety of colors in their colored state, including yellow, red, blue and blue-black. Among their drawbacks are modest light/dark contrast, large and asymmetric switching voltages, and modest cyclability. These polymers are generally not transparent in their light state, but rather lightly colored, semi-translucent, the colors varying from undesirable reds, yellows and blues to desirable grays.
With respect to the search for better cathodically coloring polymers, then, the propylene analogues of PEDOT, derivatives of poly(3,4-propylenedioxythiophene) (PProDOT), show improved electrochromic performance over PEDOT derivatives. Welsh et al. (1999) describe a dimethyl-substituted derivative of PProDOT with high light/dark contrast, with claimed Delta-% T ca. 65% at ca. 610 nm (the wavelength of highest absorption of the polymer); their Delta-% T numbers are however of electrochromic devices incorporating the polymer which are subtracted for the absorption of the substrates, i.e. they give the absorption due to the polymer alone, with the substrates rather than air used as reference; based on the expected absorptions for the substrates they use, the Delta-% T for the dimethyl-substituted PProDOT is closer to 38% for the actual device against air (rather than substrate) reference. Nevertheless, Welsh et al. demonstrate, in a comparison of the electrochromic properties of the dimethyl-PProDOT with the unsubstituted PProDOT that the substitution, in this case 2,2′ dimethyl substitution, on the propylene of the ProDOT monomer yields significant improvement of the electrochromic properties of the resulting polymer, such as improved light/dark contrast and a lower and more symmetric switching voltage (in the case of dimethyl-PProDOT, a convenient ca. +/−1.0 V).
Krishnamoorthy et al. disclose dibenzyl-substituted derivatives of PProDOT, which are also cathodically coloring conducting polymers; these appear to the best reported electrochromic performance to date for cathodically coloring conducting polymers, although again, the data are quoted vs. substrate rather than air reference so actual performance must only be estimated. The wavelength of highest absorbance of this polymer in its dark state is ca. 630 nm. Switching times of <5 seconds are reported. An advantageous feature of this polymer is that, like its dimethyl-substituted analog (Welsh et al., 1999, discussed above), it switches at low, symmetrical voltages, about +/−1.0 V. This dibenzyl PProDOT (“P(DiBz-ProDOT)”) thus appears to be very well suited for use as the cathodically coloring counterpart in a complimentary-polymer electrochromic device also incorporating a well-performing anodically coloring polymer. Its wavelength of highest absorbance (630 nm) is a little on the higher wavelength side, close to the near-IR; if this could be shifted to near 550 nm, more towards the green, perhaps by a fortuitous substitution on the benzyl ring, it would constitute an ideal cathodically-coloring polymer.
Complimentary Electrode (e.g. Dual Polymer) Electrochromic Devices
Electrochromic devices incorporating complimentarily-coloring (i.e., anodically and cathodically coloring) electrochromic materials may show improved performance over devices containing a single (either anodically or cathodically coloring) electrochromic material. Set forth here now are examples of such improved performance in actually reported data to date.
For example, a complimentary electrochromic device based on poly(o-methoxyaniline) doped with p-toluene sulfonic acid (PoANis-TSA) as the anodically coloring polymer and a blend of poly(4,4′-dipentoxy-2,2′-bithiophene) (PET2) and poly(epichlorohydrin-co-ethylene oxide) (Hydrin-C) is described in a publication of Gazotti et al. (1998). In this device, moderate light/dark contrast, Delta % T=32% at 620 nm (though again vs. a substrate reference rather than an air reference) is coupled with very fast switching time, <2 seconds, as is to be expected for such a complimentary polymer device based on the discussion above. As another example, complimentary polymer devices based on co-polymers of ethylene-dioxythiophene derivatives with N-methylcarbazole are described in a publication of Sapp et al. In this work, twelve complimentary polymer pairs are studied, all having EDOT derivatives as the cathodically coloring component. The best switching time reported in this work is ca. 3 seconds and the best light/dark contrast, Delta-% T, of 63% at 650 nm, the wavelength of highest absorbance (although this is again with device substrate rather than air as reference): A correction for the substrate absorption yields a corrected Delta-% T of 40% (vs. 63% uncorrected). Additionally, the very high wavelength of highest absorption (650 nm, in the red and close to the near-IR boundary) and the narrow rather than broad-band nature of the absorption is a serious drawback of the best of these 12 complimentary-polymer devices. In another example, Groenendal et al. claim light/dark contrasts as high as 45% at 620 nm for one P(EDOT) polymer in a complimentary polymer device; again, however, these values represent substrate-subtracted spectra, and actual contrasts (i.e. against air reference) are closer to 30% for this polymer.
In yet another example of complimentary-electrochromic devices U.S. Pat. No. 6,859,297 (2005), issued to Lee et al., discloses an amorphous, anodically coloring electrochromic material comprising nickel oxide doped with tantalum. This material is deposited on a transparent, conductive substrate. Notably, it is coupled with a cathodically coloring material, such as electrochromic material based on tungsten oxide, yielding a complimentary-electrochromic device having cathodically and anodically electrochromic materials in the same device. The composite device is shown to be significantly superior in performance to single-electrochromic (either cathodically or anodically coloring) devices.
The complimentary-polymer electrochromic devices and systems discussed above however, have very significant drawbacks. The first of these drawbacks is that the complimentary polymers are not well matched in terms of the potential at which they undergo oxidation/reduction. As an example, in its cyclic voltammogram, the cathodically-coloring poly(isothianapthene) shows two sharp oxidation peaks between +0.5 and +1.2 V, a reduction peak at ca. +0.8 V, and another reduction peak at ca. +0.4 V, all vs. Ag/AgCl (Chandrasekhar, 1990). In comparison, the anodically-coloring poly(diphenyl amine) and poly(4-amino-biphenyl) both show oxidation peaks at ca. +0.5 V and ca. +0.8 V and reduction peaks at ca. +0.8 V and +0.5 V (all vs. Ag/AgCl) (Guay et al. 1989). Similarly, the anodically-coloring poly(N,N′-diphenyl benzidine) shows a single oxidation peak at ca. +1.4 V and a single reduction peak at ca. 0.0 V (all vs. Ag/AgCl) (Chandrasekhar et al., 1991). Thus, even with a small shift expected in dual-polymer devices, these anodically-coloring polymers would make a very poor match for the cathodically-coloring poly(isothianaphthene). When the anodically coloring polymer of the pair is fully oxidized at the most extreme (+) voltage usable for the pair, the cathodically coloring polymer may only be partially reduced and so not able to contribute fully to the electrochromic contrast. Indeed, such a “mismatch” situation for most prior-art cathodically-coloring and anodically-coloring polymers may be demonstrated by experiment.
A second drawback of these complimentary-polymer systems is that nearly all of the cathodically-coloring polymers used do not themselves (i.e. on their own, in single-polymer devices) show significant light/dark contrast; they also frequently show narrow-band absorption. On the rare occasions that a high-contrast cathodically coloring polymer, such as the dibenzyl-PProDOT (P(DiBz-ProDOT)) referenced above, has been used in a complimentary polymer device, it has been paired with poorly matched anodic conducting polymers which also display mediocre electrochromic performance. See, e.g., Invernale et al. (2009) and Padilla et al. (2007). Additionally, nearly all cathodically-coloring polymers used in such devices are not transparent in their light state, but rather translucent, with significant, sometimes undesirable (e.g. light green or blue) coloration. Yet further, except in rare cases such as the P(DiBz-ProDOT) cited above, cathodically-coloring polymers used in complimentary devices to date generally have narrow band absorption which is frequently in the red region, with the wavelength of highest absorption generally in the 620 to 650 nm range.
A third drawback of these prior art complimentary-polymer systems, related to the first two, is that the redox reactions of the pair are not matched in terms of number of electrons involved. For example in the cited study of Sapp et al. (1998), the anodically coloring redox reaction in many of the pairs studied is a 2-electron reaction whilst the cathodically coloring reaction is a 1-electron reaction. Such a mismatch generates significant overpotential which reduces the electrochromic efficiency of the device.
Accordingly, there is a significant need in the art for dual-polymer devices that are capable of overcoming the aforementioned deficiencies.